Electron Paramagnetic Resonance (EPR) is widely employed in various applications in the fields of physics, chemistry, medicine and biology. Historically, EPR was used primarily to study samples doped with paramagnetic ions, rather than substantially pure samples. In order to study the samples whose physical properties are unaffected by dopant impurities, particularly samples of materials with low concentration of paramagnetic centers, however, the spectrometer sensitivity, i.e. its signal-to-noise (S/N) ratio had to be increased.
One of the conventionally used techniques of increasing signal-to-noise (S/N) ratio of the EPR spectrometer entails placing the dielectric material inside a resonator. For example, disposing a quartz plate proximal to the sample in one instance increased the S/N ratio about 4.5 times. Further, increasing intensity of the EPR signal can be obtained by using a ferroelectric material as a microwave resonator. For instance, rutile (TiO2) rectangular resonator has been used to increase Fe3+ EPR spectrum. Dielectric resonators fabricated from TiO2 and SrTiO3 (these materials have anisotropic permittivity) were used only for increasing the EPR signal intensities of paramagnetic centers inside themselves.
Conventional EPR spectrometers generally include a high Q resonant metal cavity. The high Q cavity requires a high ratio of the cavity volume to the cavity wall surface, which, in turn, necessitates a relatively large volume of polarizing magnetic field, thereby requiring a large magnet, e.g. the magnet weighing 1000 kg or more. These spectrometers have been designed as highly sensitive and versatile instruments for a broad variety of applications. As a result, conventional spectrometers are relatively expensive and bulky, and are typically used as stationary equipment in research laboratories.
Other techniques rely on inserting a ferroelectric object, i.e. a resonator, into a standard cavity of the EPR spectrometer to alter intensities of a continuous wave (“CW”) EPR signal and Pulse EPR echo (see, e.g., an article by I. N. Geifman, I. S. Golovina, V. I. Kofman, and E. R. Zusmanov, Ferroelectrics, Vol. 234 (1-4), pp. 81-88 (1999), incorporated herein by reference). Thusly configured resonating structures are suitable for analyzing a wide variety of materials. Ferroelectric KTa03 resonators described by Geifman and his co-authors are capable of increasing S/N ratio ten times at room temperature for a rectangular resonator in CW EPR experiments and reducing microwave power by a factor of 50 at 50 K in an electron spin echo (ESE) experiment. Among other advantages, these resonators have isotropic dielectric constant, and low dielectric losses. Although these ferroelectric resonators have increased sensitivity, as well as much smaller dimensions in comparison with the conventional dielectric resonators mentioned above and can be utilized with liquid and solid samples of a wide range of materials, they still operate as mere “amplifiers” within the conventional EPR cavity and, actually, is a feature of the sample rather than a part of the spectrometer.
Some conventional EPR spectrometers, e.g. the one disclosed in U.S. Pat. No. 3,931,569, were designed from the point of view of arriving at a high sensitivity instrument for examining aqueous samples or other liquid samples of relatively high dielectric loss at room temperature. As is well known, the polarizing magnetic field for electron paramagnetic resonance is parallel to a cavity dimension that corresponds to the zero index, and the magnitude of this cavity dimension does not affect the resonant frequency of the cavity. In this case, the cavity dimension corresponding to the zero index is made small, of the order of an optimum capillary sample tube outer diameter, i.e., 2 mm if the diameter is 1 mm, for apparatus designed to operate at 10 GHz. It has been found that the sensitivity of the spectrometer is as high as that obtained from high Q cavities of conventional design where the dimension corresponding to the zero index is typically 15 mm at 10 GHz. The design is directed particularly to EPR spectrometers utilizing rectangular TE102 or cylindrical TM110 modes and provides low-cost apparatus for a very restricted range of applications. The low Q resonant cavity can be applied to analyze aqueous samples only.
Although Nuclear Magnetic Resonance (NMR) devices, due to frequency range lower relative to EPR, have a radio frequency (RF) coil instead of the cavity as the resonator, fundamental dependencies remain between the resonator size and the device capabilities, complexity, size, and cost. The dependencies equally affect NMR spectrometers, MASERs, probes and, generally, any device with the RF coil.
NMR spectrometers have now become very complex instruments capable of performing sophisticated experiments. However, conceptually, broken down to its simplest form, the spectrometer consists of an intense, homogeneous, and stable magnetic field; a “probe”, which enables RF coils to be placed close to a sample; a RF transmitter capable of delivering short pulses; a receiver to amplify the NMR signals; a digitizer to convert the NMR signals into a form, which can be stored in computer memory; a “pulse programmer” to produce precisely timed pulses and delays; as well as means for controlling and processing data.
As described in more detail below, in its various embodiments, the present invention relates to the probe of the NMR spectrometer. As the static magnetic field generating magnet in the NMR spectrometer, a resistive magnet of 0.5 to 2.2 T or a superconducting magnet of 0.5 to 18.8 T, as well as, in some cases, a permanent magnet have been used. Modern NMR spectrometers use persistent superconducting magnets to generate the B0 field. Basically such a magnet consists of a coil of wire through which a current passes, thereby generating a magnetic field. The wire is of a special construction such that at low temperatures (less than 6 K, typically) the resistance goes to zero, so the wire is superconductor. Thus, once the current is set running in the coil it will persist for ever, thereby generating a magnetic field without the need for further electrical power. Superconducting magnets tend to be very stable and so are very useful for NMR. To maintain the wire in its superconducting state the coil is immersed in a bath of liquid helium. Surrounding this is a “heat shield” kept at 77 K by contact with a bath of liquid nitrogen; this reduces the amount of expensive liquid helium, which boils off due to heat flowing in from the surroundings. The whole assembly is constructed in a vacuum flask so as to further reduce the heat flow.
There is a sample region accessible to the outside environment. The region has to be engineered as part of the design of the magnet and, conventionally, it takes the form of a vertical tube passing through the magnet (called the bore tube of the magnet); the magnetic field is in the direction of this tube.
The lines in NMR spectra are very narrow. Line widths of 1 Hz or less are not uncommon so the magnetic field has to be extremely homogeneous for work at this resolution. For example, a proton spectrum recorded at 500 MHz requires variations, which, expressed as a fraction of the main magnetic field, are no more than 2×10−10. On its own, no superconducting magnet can produce such a homogeneous field. Thus the sample is surrounded with a set of shim coils, each of which produces a tiny magnetic field with a particular spatial profile. The current through each of these coils is adjusted until the magnetic field has the required homogeneity. Essentially, the magnetic fields produced by the shims are canceling out the small residual variations in the main magnetic field. Modern spectrometers might have up to 40 different shim coils, so adjusting them is a very complex task. Moreover, even after set on installation, it is usually necessary on a day-to-day basis to alter a few of the shims. The shims are labeled according to the magnetic field profiles they generate. The field profiles that the shims coils create are, in fact, the spherical harmonic functions, which are the angular parts of the atomic orbital.
The probe is a cylindrical metal tube, which is inserted into the bore tube of the magnet. Small RF coil used to both excite and detect the NMR signal is held in the top of this assembly in such a way that the sample can come down from the top of the magnet and drop into the coil. Various other pieces of electronics are contained in the probe, along with some arrangements for heating or cooling the sample. The key part of the probe is the small coil used to excite and detect the magnetization. To optimize the sensitivity this coil needs to be as close as possible to the sample. Extraordinary effort has been put into the optimization of the design of this coil.
The coil forms a part of a tuned circuit consisting of the coil and a capacitor. The inductance of the coil and the capacitance of the capacitor are set such that the circuit they form is resonant at the Larmor frequency. “Tuning the probe” means adjusting the capacitor until the tuned circuit is resonant. Usually, it is also needed to “match the probe”, which involves further adjustments designed to maximize the power transfer between the probe and the transmitter and receiver. The two adjustments tend to interact rather, so tuning the probe is a tricky process. To aid it, the instrument manufacturers provide various indicators and displays so that the tuning and matching can be optimized. The tuning of the probe is particularly sensitive to changing solvent or to changing the concentration of ions in the solvent.
Ferroelectric crystals are materials, characterized by high dielectric constant (ε). Certain binary metal oxides, particularly oxides of combinations of alkali with group (V) metals and of alkaline earth with group (IV) metals are ferroelectrics. A potassium tantalate crystal is unique among ferroelectrics because it combines, in spite of the lowering temperature, the rising considerably isotropic dielectric constant, which reaches 4000 at approximately 4.2° K. with decreasing dielectric losses. These features provide for resonators of high quality (Q) even under super low temperature conditions. The potassium tantalate single crystal is the promising material for radio frequency and microwave resonators and, as a result, for EPR and NMR applications including spectroscopy and imaging (MRI).
It has been known that ferroelectrics in general and especially a potassium tantalate single crystal can improve an EPR spectrometer performance that provides for a possibility of decreasing size and cost of spectrometers. However, a single crystal of the nominally pure potassium tantalate most often shows EPR spectra of iron (Fe3+), which is present in the crystal as an uncontrolled impurity. Therefore, when used as a resonator, the crystal carries own background EPR signal that overlaps sample spectra. Furthermore, other possible impurities and structural defects further limit usefulness of the crystal.
An EPR resonator made of single crystal potassium tantalate doped with lithium (Li), which replaces 0.01-0.1% of potassium disclosed in UA Pat. No. 40178A. The crystal characterized by absence of the background signal, decreasing irregularities, and stronger crystal lattice. Nonetheless, the resonator is of limited use for EPR applications because, at low temperatures, the crystal walls surrounding a central hole for a sample are becoming too thin to sustain stresses of a microwave field. Alternatively, NMR applications, due to lower frequency range, require radically greater size of the resonator, its ε, or both.
Traditionally, variations of Czochralski method is used for growing sizable ferroelectric single crystals in general and potassium tantalate single crystals in particular. Different means are employed for achieving desired crystal parameters. Particularly, controlling melt component relative quantities, temperature, temperature gradient, flow direction and intensity; growing in the air, special atmosphere, or vacuum; manipulating seed lifting speed, rotational direction and velocity have been known in the art.
However, almost each chemical composition requires its own set of conditions for growing single crystals of satisfactory quality. Finding right conditions for growing a particular crystal remains as much an art as a science. Thus, there remains an unresolved need in the art for an improved method of producing ferroelectric potassium tantalate single crystal having cubic form of perovskite crystalline structure and is essentially free of impurities and defects.
Thus, there remains an unresolved need in the art for improved EPR and NMR frequency resonators as well as for improved devices that utilize EPR and NMR methods, in particular EPR and NMR spectrometers.